The Content Scramble System (CSS) and the Content Protection for Recordable Media (CPRM) are used to protect illegal copy of content. The CSS protects content recorded on as read-only memory (ROM) type recording medium, such as a digital versatile disc (DVD) or the like, by scrambling the content so that the content cannot be played back. On the other hand, the CPRM protects content recorded on a recordable recording medium.
However, both the CSS and the CPRM are not enough to prevent illegal copy of content recorded on a DVD. In other words, above-mentioned conventional technologies cannot sufficiently respond to an issue of copyright piracy that has been taking place frequently in recent years.
Namely, there are reports that a DVD player is illegally altered to play back DVD content and the DVD content is unscrambled and converted to plaintext content. Then, the plaintext content, which is not scrambled, can be easily copied onto other DVDs or the like. The copied DVD, so-called “pirated DVD”, is illegally distributed. The pirated DVD can also be widely distributed by peer-to-peer file sharing via the Internet.
To solve the problem, Advanced Access Content System (AACS) is developed for next-generation DVD-videos such as high-definition (HD) DVDs. As disclosed in AACS Pre-recorded Video Book Revision 0.90 (Chapters 2 and 3), the AACS employs a Content Certificate (CC) technology to detect whether content is illegally recorded on a recording medium even if the content is already converted to plaintext content.
In the CC technology, a hash value is generated from a portion of content to create a content certificate therein. The content certificate contains a signature that is encrypted by using a private key held by a licenser. A predetermined digital watermark (WM) is embedded in the content when the content is produced. The WM-embedded content is encrypted and recorded together with the content certificate on a recording medium. The WM is set to abort playback of the content whenever the WM is detected. On the other hand, a public key corresponding to the private key included in the content certificate is embedded in a content playback apparatus (e.g., a DVD player). When the content is to be played back, before playing back the content, the content playback apparatus checks whether the content certificate contains an appropriate signature. Moreover, during playback of the content, the content playback apparatus calculates a hash value of the content and checks whether the hash value is not altered based on comparison of the calculated hash value with the hash value contained in the content certificate. If a recording medium does not contain a content certificate, or if the content certificate included in the content does not include an appropriate signature, the content playback apparatus during playback checks whether a WM is present in the content. When a WM is detected, the content playback apparatus is set to abort playback. Thus, the DVD player can prevent playback of a pirated DVD.
Even if encrypted content is unscrambled and converted to plaintext content by an illegally-altered content playback apparatus, and the plaintext content is copied on a recording medium, a content certificate including a signature cannot be recorded on the plaintext content properly. Thus, a legitimate content playback apparatus will confirm presence of a WM in content and successfully abort playback of the content. Also, even if the plaintext content is further encrypted by an illegally-altered content playback apparatus to disguise the content as proper encrypted content, it is not possible to forge a signature because only the private key held by the licenser can be used to create the signature. Thus, if a legitimate content playback apparatus is used to back the illegally-copied content, the legitimate content playback apparatus cannot successfully verify a content certificate, and also detects a WM in the content, whereby the legitimate content playback apparatus aborts playback of the content. A detection of a content certificate is set to be performed on video content, i.e., a ROM medium (a pre-recorded medium) only. Therefore, even if a content certificate is copied from a proper ROM medium onto a recordable recording medium, a legitimate DVD player can abort playback of illegally-copied content by detecting a WM embedded in the content despite the content contains the content certificate.
If a user is allow to copy content upon observance of predetermined rules in the near future, the user can copy the content onto a hard disk drive (HDD) or the like so that the user enjoys playing back the content outside without physically carrying the content. Also, the user can back up the content in case a source recording medium is physically damaged.
However, the conventional CC technology covers exclusively for content recorded on a ROM medium. Thus, it is not possible to record a content certificate on a recordable recording medium in a legitimate manner. Moreover, it is not possible to detect whether content recorded on a recordable recording medium is illegally copied during playback.